These days, there is an increasing demand for a semiconductor device as a power module, which includes a plurality of semiconductor elements generating large amount of heat and is configured with a smaller body, on a trend of downsizing electronic appliances.
An existing semiconductor device (power module) is explained below with reference to FIGS. 1 and 2. A power module 51 includes a plurality of semiconductor elements. Additionally, a thermal conductor base board 52, which is shaped like a board with a ceramic material that is light in weight and has a good thermal conductivity, is attached to the bottom of the power module 51 as a thermal conductor member.
Substrates 55 and 56 are arranged on the top surface of the thermal conductor base board 52. The substrate 55 is configured by a conductor layer 55a and an insulator layer 55b, and the insulator layer 55b contacts with the thermal conductor base board 52. In the meantime, the substrate 56 is configured by a conductor layer 56a and an insulator layer 56b, and the insulator layer 56b contacts with the thermal conductor base board 52. A plurality of semiconductor elements 57 are arranged on the top surface of the conductor layer 55a, whereas a plurality of semiconductor elements 58 are arranged on the top surface of the conductor layer 56a. Here, the semiconductor elements 57 and 58 are MOSFETs, respectively. Additionally, the drain of the MOSFET is formed on one side of each of the semiconductor elements 57 and 58, whereas the source and the gate of the MOSFET are formed on the other side. The drain of each of the semiconductor elements 57 is in contact with the conductor layer 55a of the substrate 55, whereas the drain of each of the semiconductor elements 58 is in contact with the conductor layer 56a of the substrate 56.
A substrate 53 is arranged in the central region of the top surface of the thermal conductor base board 52. The substrate 53 is configured by a conductor layer 53a and an insulator layer 53b, and the insulator layer 53b contacts with the thermal conductor base board 52. Additionally, a source-drain electrode 54 is connected to the conductor layer 53a. 
The substrates 55, 56, and 53 (and the semiconductor elements 57 and 58) are surrounded by a resinous case 59. A drain electrode 60, a source electrode 61, and gate electrodes 62 and 63 are attached to the resinous case 59. Additionally, the resinous case 59 is fixed to the thermal conductor base board 52.
Furthermore, as shown in FIG. 1 or FIG. 2, the connection between the drain electrode 60 and the conductor layer 55a of the substrate 55, the connection between the source of each of the semiconductor elements 57 and the conductor layer 53a of the substrate 53, the connection between the conductor layer 53a and the conductor layer 56a of the substrate 56, the connection between the source of each of the semiconductor elements 58 and the source electrode 61, the connection between the gate of each of the semiconductor elements 57 and the gate electrode 62, and the connection between the gate of each of the semiconductor elements 58 and the gate electrode 63 are respectively made by wire bonding.
If the substrates 53, 55, and 56, which occupy the most of the top region of the power module 51, are formed to be small in the semiconductor device having the above described configuration, a power module of a smaller size can be manufactured. However, the region required for wire bonding, and the region for arranging the source-drain electrode 54 must be secured for the substrate 53. Additionally, to implement the large capacity of the power module 51, the numbers of semiconductor elements 57 and 58 must be increased, which also requires the substrates 55 and 56 to be formed larger than a predetermined size. Accordingly, there are limits to the downsizing of the substrates 53, 55, and 56. Namely, it is not easy to reduce the size of a semiconductor device, such as a power module, etc.